1. Field of the Invention
The present invention relates to lighting systems wherein power to a plurality of lighting fixtures is provided from a central high-frequency voltage source.
2. Description of Prior Art
Lighting systems wherein a plurality of lighting fixtures are powered from a single central source of high-frequency voltage have been described in prior patents. For instance, such systems are described in U.S. Pat. No. 3,710,177 to Richard Ward and in U.S. Pat. No. 4,207,498 to Joel S. Spira.
In such a system, one of the most efficient and cost-effective ways of ballasting a gas discharge lamp from the central source of high-frequency voltage is by way of a series-resonant circuit; that is, by connecting the series-resonant circuit across the high-frequency voltage supply while connecting the lamp load in parallel with one of the reactive elements (typically the capacitor) of the series-resonant circuit.
However, there is an important problem associated with such a series-resonant ballasting method; namely, with the lamp load removed, the current drawn by the then unloaded series-resonant circuit gets to be so high as to cause destruction of the circuit itself and/or of the high-frequency voltage source.
This problem has been partly recognized, and--in the case of four-terminal fluorescent lamps--a partial solution has been described by Ward and others, a solution wherein the series-resonant circuit is automatically disconnected from the high-frequency voltage source by removing the lamp from even one of its socket terminals.
However, it is apparently not recognized that the lamp load may in effect be removed even if the lamp remains in its socket. In particular, this type of situation is apt to occur toward end of lamp life: the cathodes may then still be intact in the sense of being capable of conducting current, but the lamp may fail to start; which implies that the series-resonant circuit is still connected across the high-frequency voltage source, but the circuit loading is effectively non-present. Thus, toward end of lamp life, a potentially destructive mode of operation is apt to result.
Moreover, the partial solution described by Ward and others, is non-applicable to situations where two-terminal gas discharge lamps are used.